In recent decades, high-belite calcium sulfoaluminate cement, which combines calcium sulfoaluminate of high early strength and belite, has become a hot research topic, since it has advantages of low energy consumption, of low carbon emission and of high performance. Many universities, institutions and companies have put a lot of effort into the research in high-belite calcium sulfoaluminate cement and achieved many research developments. Yet, there is no mass production of high-belite calcium sulfoaluminate cement. According to the literature, most high-belite calcium sulfoaluminate cement clinkers have a typical oxide composition by weight of about 10% Al2O3 and about 20% SiO2. These high-belite calcium sulfoaluminate cement clinkers develop much lower early and later compressive strengths of cement paste than conventional calcium sulfoaluminate cement clinkers. This is because the raw meal design of high-belite calcium sulfoaluminate cement clinker follows the theory for calcium sulfoaluminate cement clinker; lime saturation factor Cm is set to be 1.0 and aluminum-to-sulfur ratio P is set to be 3.82, as for calcium sulfoaluminate cement clinker. Therefore, it is difficult to produce high-strength high-belite calcium sulfoaluminate cement clinker by using low-grade raw materials of high silicon and low aluminum. As the rapid development of China's industry, a large amount of industry wastes are disposed of in landfills or ponds, which causes serious environmental problems and a waste of resources High-belite calcium sulfoaluminate cement can consume various kinds of industry wastes in large quantity, and it is an innovative concept to produce new type of cement by using industry wastes. It is significant to improve the adaptability of raw materials and to product development in the field of high-belite calcium sulfoaluminate cement.
On the other hand, energy-saving and emission-reduction is an important state policy in China. There is an urgent need for housing industrialization. One of the core technologies is the standardization design and prefabrication of main components, such as beams, slabs and columns. The conventional prefabrication method is to use ordinary Portland cement and to steam-cure concrete. This method has several disadvantages of high energy cons wnption, of heavy pollution, of high labor intensity and of low production efficiency. To develop a low-cost rapid-hardening cement is significant for the sustainable development of construction industry by skipping steam curing, particularly with steam boiler using coal, by enabling automated production, by improving production efficiency and by energy-saving and emission-reduction.
Calcium sulfoaluminate cement is a special cement invented by Chinese researchers. The major constituent phases are calcium sulfoaluminate (C4A3S) and belite (C2S); the proportion of the former is about 60% to about 70% by weight, and the proportion of the former is 20% to 30% by weight. Since calcium sulfoaluminate cement has many advantages, such as rapid hardening, high strength, expansive, good frost resistance, low permeability and good resistance to steel corrosion, and it has many unique applications in special building material and concrete products. High-quality natural gypsum and bauxite are the raw materials for manufacturing calcium sulfoaluminate cements. As bauxite is getting scarce, the cost and price of calcium sulfoaluminate cement have increased greatly, which have resulted in a decreased use of the cement in engineering projects. Many manufacturers are facing operation difficulties, and there is an urgent need for developing a low-cost high-performance calcium sulfoaluminate cement, High-belite calcium sulfoaluminate cement has advantages of low energy consumption and of low carbon emission, and it has been recently become one of the hot research topics. In addition, another advantage of high-belie calcium sulfoaluminate cement is that it can be produced by using fly ash, FGD gypsum and other industry wastes, while the manufacture of calcium sulfoaluminate cement need high-quality gypsum and bauxite. As the rapid development of China's industry, a large amount of industry wastes are disposed of in landfills or ponds, which causes serious environmental problems and a waste of resources. This creates great potential to produce a new type of high-belite calcium sulfoaluminate cement by using industry wastes. The development of high-belite calcium sulfoaluminate cement is of great importance in energy-saving and emission-reduction and will bring new opportunities to cement industry.
Calcium sulfoaluminate cements was invented by the researchers at China Building Materials Academy nearly 40 years ago. Ordinary calcium sulfoaluminate cement clinker contains about 30% by weight of Al2O3 and about 10% by weight of SiO7. The experimental results from Department of Cement Research at China Building Materials Academy revealed that as the C4A3S to β-C2S ratio of calcium sulfoaluminate cement clinkers decreases from 80:20 to 60:40 to 40:60, the 3-day compressive strength of cement pastes decreases by about 10 MPa successively. This indicates that the decrease of Al2O3 content and the increase of SiO2 content in clinkers result in a large decrease in compressive strength proportionally. Using high-quality bauxite of high aluminum and low silicon is a main approach to manufacture high-quality calcium sulfoaluminate cement, which is in good agreement with the production experiences of many manufacturers. It has been become a hot research topic in the cement industry to use low-aluminum high-silicon bauxite to produce a high-quality calcium sulfoaluminate cement, which has a relatively high β-C2S content and is, therefore, called high-belite calcium sulfoaluminate cements. According to the literature, high-belite calcium sulfoaluminate cement clinkers develop much lower early and later compressive strengths of cement paste than conventional calcium sulfoaluminate cement clinkers. High-belite calcium sulfoaluminate cement develops mechanical properties relatively close to Portland cement and has no rapid-hardening feature as conventional calcium sulfoaluminate cement. The raw meal design of high-belite calcium sulfoaluminate cement clinker follows the theory for the raw meal design of calcium sulfoaluminate cement clinker; lime saturation factor Cm is set to be 1.0 and aluminum-to-sulfur ratio P is set to be 3.82. Consequently, it is difficult to produce high-strength high-belite calcium sulfoaluminate cement clinker by using low-grade bauxite of high silicon and low aluminum. The absence of a well-developed theory has led to a slow progress in this research field. In order to produce high-quality calcium sulfoaluminate cements by using industry wastes and low-grade raw materials, the conventional theory of clinker design should be improved.
P. K. Mehta from the United States has reported his research on a high-belite calcium sulfoaluminate cement clinker in “World Cement Technology”, July/August 1978, page 144-160 and in “World Cement Technology”, May 1980, page 166-177. The clinker comprises 20% by weight of ye'elimite (C4A3S), 20% by weight of calcium sulfate (CaSO4), 45% by weight of belite (C2S), 15% by weight of tetracalcium alumino ferrite (C4AF) and <0.5% by weight of f-CaO. The difference between Mehta's clinker and Chinese calcium sulfoaluminate cement clinker, called third series cement, is that the former has a relatively higher calcium sulfate content. The clinker No. 5 in Mehta's publication seemed to meet the requirement of CO2 emission reduction and developed similar properties to Portland cement. However, the clinker invented by Mehta has never been available on the market and there is no report been published on its commercialization.
Lafarge, a French company, has disclosed a high-belite calcium sulfoaluminate cement clinker in Chinese patent CN102745918A. This clinker comprises the following major constituent phases: 5% to 25% by weight of C2AXF(1-X), 15% to 35% by weight of C4A3S, 40% to 75% by weight of C2S. Some minor components are added as additives in clinker calcination to tailor the properties of the clinker and to ensure that part of C2S exists in the formation of α-C2S. According to the experimental data in the patent, the early strength is relatively low and the later strength is not high as well. There is still a large gap between the properties of the clinker and the requirements of high performance.
Calcium sulfoaluminate cement is featured by rapid hardening and high strength, and the increase in long-term strength is, however, relatively small. Also, the production needs a large amount of high-quality bauxite, gypsum, limestone and other resources. In contrast, bauxite, limestone and other raw materials are not necessarily in high quality for the production of high-belite calcium sulfoaluminate cement. However, current high-belite calcium sulfoaluminate cements have mechanical properties relatively close to Portland cement and have no features of rapid hardening and high early and later strengths as conventional calcium sulfoaluminate cements. The cement industry has been looking forward to a high-performance high-belite calcium sulfoaluminate cement, which can be manufactured by using massive industry wastes, is of low cost, of low energy consumption and of low CO2 emission and shows faster hardening and higher early and later strengths than calcium sulfoaluminate cements. This is also one of the major technique difficulties in the cement industry.